Add changelog and update documentation rules

Add a top-level changelog.md and update documentation guidance to require and describe its use. .cursor/rules/documentation-rules.mdc: expanded description and added sections for “Changelog vs Dev Log” and “Changelog Format” with Keep a Changelog guidance and an example. .cursor/rules/project-overview.mdc: reference changelog.md and clarify that docs/dev-log.md is for milestones, testing, and discoveries. changelog.md: new file with an [Unreleased] section and an initial 0.0.1 stub (notes that docs/changelog.md was moved).

.cursor/rules/documentation-rules.mdc

@@ -1,5 +1,5 @@
 ---
-description: Documentation and development log update rules
+description: Documentation, changelog, and development log update rules
 alwaysApply: true
 ---
 
@@ -8,12 +8,66 @@ alwaysApply: true
 After every meaningful code change, update the relevant documentation. Do not let code and documentation drift apart.
 
 Always consider whether these files need updates:
+- `changelog.md`
 - `docs/dev-log.md`
 - `docs/protocol.md`
 - `docs/architecture.md`
 - `docs/setup.md`
 - `docs/testing.md`
 
+## Changelog vs Dev Log
+
+Use both files, but for different purposes:
+
+| File | Purpose | When to update |
+|------|---------|----------------|
+| `changelog.md` | Complete record of every change | After **every** code edit, addition, removal, fix, or documentation update |
+| `docs/dev-log.md` | Milestones, testing, discoveries, next steps | After feature milestones or meaningful development sessions |
+
+**Changelog** — log every change under `[Unreleased]` using [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) categories. Include what was added, changed, deprecated, removed, fixed, or security-related. Mention affected files or areas when helpful. Do **not** include testing results, test plans, verification notes, known issues, or next steps.
+
+**Dev log** — keep the existing milestone format for broader feature work, technical discoveries, and testing.
+
+## Changelog Format
+
+Follow [Keep a Changelog 1.1.0](https://keepachangelog.com/en/1.1.0/).
+
+- Keep an `## [Unreleased]` section at the top for all in-progress changes.
+- When cutting a release, move `[Unreleased]` entries into a new `## [X.Y.Z] - YYYY-MM-DD` section (ISO 8601 date), then clear `[Unreleased]`.
+- Newest versions come first, directly under the file preamble.
+- Omit empty change-type sections.
+- One bullet per change; keep entries concise and factual.
+
+Use these change types (only when applicable):
+
+| Type | Use for |
+|------|---------|
+| `Added` | New features, files, fields, or capabilities |
+| `Changed` | Changes to existing functionality |
+| `Deprecated` | Soon-to-be removed features |
+| `Removed` | Removed features or files |
+| `Fixed` | Bug fixes |
+| `Security` | Vulnerability fixes |
+
+Example:
+
+```md
+## [Unreleased]
+
+### Added
+- New packet field `exampleField` in transform sync.
+
+### Fixed
+- Guard null actor pointer before proxy movement update.
+
+## [0.1.0] - 2026-06-23
+
+### Added
+- Initial world-state sync MVP.
+```
+
+## Dev Log Format
+
 At minimum, after every milestone or feature change, update `docs/dev-log.md` with:
 - Date
 - Summary of what changed

.cursor/rules/project-overview.mdc

@@ -18,7 +18,8 @@ Important project areas:
 - `server/server.py`: Python relay server.
 - `server/fake_client.py`: fake testing client for protocol and relay checks.
 - `docs/`: architecture, setup, protocol, testing, and milestone documentation.
-- `docs/dev-log.md`: running project history and development log.
+- `changelog.md`: dated record of every code and documentation change (no testing notes).
+- `docs/dev-log.md`: milestone history, testing results, discoveries, and next steps.
 
 Cursor workflow:
 - Read relevant files before editing.